In the process of combustion of liquid or gaseous fuels in combustion apparatus, the flow of combustion air must be controlled in relation to fuel flow to maintain the most efficient practical air to fuel ratio.
A common way of achieving air/fuel ratio control is by interconnecting the devices which regulate the flow of air and fuel with a mechanical linkage which is calibrated to provide the desired air to fuel ratio throughout the operating range of the combustion apparatus. Typically a jackshaft system is employed for maintaining a predetermined air to fuel ratio in prior art combustion control apparatus.
A disadvantage of the jackshaft system is that no convenient means is available for compensating the initial air to fuel calibration for changing conditions such as heating value of the fuel, air density and humidity, draft losses, burner wear, and the like. If, at a given operating point one were to adjust the linkages of the jackshaft system to obtain a specific air to fuel ratio at that point, it is most likely that this adjustment would result in the air to fuel ratio being incorrect at many of the other load points. To make an adjustment which will be substantially correct throughout the load range requires that the position of the air and fuel control devices be accurately established under the desired operating conditions at representative load points throughout the operating range, and the interconnecting linkage be recalibrated to provide this position relationship at those load points.